Potential for methane release

This post was written by Sam Carana on December 20, 2011, and was updated January 29, 2012.
It is preserved here for archival purposes. For an update, see the The Threat.

The potential for methane releases in the Arctic to cause runaway global warming

What are the chances of abrupt releases of, say, 1 Gt of methane in the Arctic? What would be the impact of such a release?

How much methane is there in the Arctic?

An often-used figure in estimates of the size of permafrost stores is 1672 Gt (or Pg, or billion tonnes) of Carbon. This figure relates to organic carbon and refers to terrestrial permafrost stores. (1)

This figure was recently updated to 1700 Gt of carbon, projected to result in emissions of 30 - 63 Gt of Carbon by 2040, reaching 232 - 380 Gt by 2100 and 549 - 865 Gt by 2300. These figures are carbon dioxide equivalents, combining the effect of carbon released both ascarbon dioxide (97.3%) and as methane (2.7%), with almost half the effect likely to be from methane. (2)

In addition to these terrestrial stores, there is methane in the oceans and in sediments below the seafloor. There are methane hydrates and there is methane in the form of free gas. Hydrates contain primarily methane and exist within marine sediments particularly in the continental margins and within relic subsea permafrost of the Arctic margins. (3)

Hunter and Haywood estimate that globally between 4700 and 5030 Pg (Gt) of Carbon is locked up within subsea hydrate within the continental margins. This does not include subsea permafrost-hosted hydrates and so those of the shallow Arctic margin (<~300m) were not considered. (3)

Dallimore and Collett (1995) found high methane concentrations in ice-bonded sediments and gas releases suggest that pore-space hydrate may be found at depths as shallow as 119 m. (4) Recent studies indicate that hydrate formation can occur in upper gas-saturated horizons (up to 100-200 m) of permafrost. (5) Furthermore, methane hydrates have been found in Siberia at depths as shallow as 20 m. (6)Shakhova et al. estimate the accumulated methane potential for the Eastern Siberian Arctic Shelf (ESAS, rectangle on image right) alone as follows:

The East Siberian Arctic Shelf covers about 25% of the Arctic Shelf (3) and additional stores are present in submarine areas elsewhere at high latitudes. Importantly, the hydrate and free gas stores contain virtually 100% methane, as opposed to the organic carbon which the above study (2) estimates will produce emissions in the ratio of 97.3% carbon dioxide and only 2.7% methane when decomposing.

How stable is this methane?

It does take time for heat to be transferred down sediments. What can take place much more rapidly, though, is for heat to be transferred down fluids in cracks and openings in the rock and sediment, called pingos.

The image right, from Hovland et al., shows pingo-like sediment features, formed by local accumulation of hydrate (ice) below the sediment surface, and methane migrating upwards through conduits. (8)

A recent study by Serié describes geophysical signatures of different development stages associated with the formation and dissociation of shallow gas hydrate, as well as their link to deep-rooted plumbing systems that allow thermogenic fluid migration from several-kilometers-deep sedimentary basins. (9)

Paull et al. describe pingo-like-features on the Beaufort Sea Shelf, adding that a thermal pulse of more than 10 degrees Celsius is still propagating down into the submerged sediment and may be decomposing gas hydrate as well as permafrost. (10)

The sensitivity of gas hydrate stability to changes in local pressure-temperature conditions and their existence beneath relatively shallow marine environments mean that submarine hydrates are vulnerable to changes in bottom water conditions (i.e. changes in sea level and bottom water temperatures). Following dissociation of hydrates, sediments can become unconsolidated, and structural failure of the sediment column has the potential to trigger submarine landslides and further breakdown of hydrate. The potential geohazard presented to coastal regions by tsunami is obvious. (3)

Further shrinking of the Arctic ice-cap results in more open water, which not only absorbs more heat, but which also results in more clouds, increasing the potential for storms that can cause damage to the seafloor in coastal areas such as the East Siberian Arctic Shelf (ESAS, rectangle on image left), where the water is on average only 45 m deep. (11)

Much of the methane released from submarine stores is still broken down by bacteria before reaching the atmosphere. Over time, however, depletion of oxygen and trace elements required for bacteria to break down methane will cause more and more methane to rise to the surface unaffected. (12)

There are only a handful of locations in the Arctic where (flask) samples are taken to monitor the methane. Recently, two of these locations showed ominous levels of methane in the atmosphere (images below).

The danger is that large abrupt releases will overwhelm the system, not only causing much of the methane to reach the atmosphere unaffected, but also extending the lifetime of the methane in the atmosphere, due to hydroxyl depletion in the atmosphere.

Shakhova et al. consider release of up to 50 Gt of predicted amount of hydrate storage as highly possible for abrupt release at any time. (13)

What would be the impact of methane releases from hydrates in the Arctic?

If an amount of, say, 1 Gt of methane from hydrates in the Arctic would abruptly enter the atmosphere, what would be the impact?

Methane's global warming potential (GWP) depends on many variables, such as methane's lifetime, which changes with the size of emissions and the location of emissions (hydroxyl depletion already is a big problem in the Arctic atmosphere), the wind, the time of year (when it's winter, there can be little or no sunshine in the Arctic, so there's less greenhouse effect), etc. One of the variables is the indirect effect of large emissions and what's often overlooked is that large emissions will trigger further emissions of methane, thus further extending the lifetime of both the new and the earlier-emitted methane, which can make the methane persist locally for decades.

The IPCC (2007) gives methane a lifetime of 12 years, and a GWP of 25 as much as carbon dioxide over 100 years and 72 as much as carbon dioxide over 20 years. (14)

The image by Dessus (2008) below illustrates how methane's GWP depends on the horizon over which its impact is calculated. (15)

Drew Shindell (2009) points out that the IPCC figures do not include direct+indirect radiative effects of aerosol responses to methane releases that increase methane's GWP to 105 over 20 years when included. (16)

Using the IPCC figures, applying a GWP of 72 times carbon dioxide would give 1 Gt of methane a greenhouse effect equivalent to 72 Pg of carbon dioxide over 20 years. Applying a GWP of 105 times carbon dioxide would give 1 Gt of methane a greenhouse effect equivalent to 105 Pg of carbon dioxide over 20 years.

Note that this 174 Pg C should be multiplied by 3.667 to get units of carbon dioxide, as in above graph.

The image on the left shows the impact of 1 Gt of methane, compared with annual fluxes of carbon dioxide based on the NOAA carbon tracker. (18)

Globally, 9.139 Pg C was emitted from fossil-fuel combustion and cement manufacture in 2010. Converted to carbon dioxide, so as to include the mass of the oxygen molecules, this amounts to over 33.5 Gt of carbon dioxide. (19)

Fossil fuel and fires have been adding an annual flux of just under 10 Pg C since 2000 and a good part of this is still being absorbed by land and ocean sinks.

In other words, the total burden of all carbon dioxide emitted by people since the start of the industrial revolution has been partly mitigated by sinks, since it was released over a long period of time.

Furthermore, the carbon dioxide was emitted (and partly absorbed) all over the globe, whereas methane from such abrupt releases in the Arctic would - at least initially - be concentrated in a relatively small area, and likely cause oxygen depletion in the water and hydroxyl depletion in the atmosphere, extending methane's lifetime, while triggering further releases from hydrates in the Arctic.

This makes it appropriate to expect a high initial impact from an abrupt 1 Gt methane release, i.e. at a GWP of well over 100 times the greenhouse effect of carbon dioxide, which will last for decades.

Even more terrifying is the prospect that this would trigger further methane releases. Given that there already is ~5 Gt in the atmosphere, the impact of this initial 1 Gt combined with further releases of, say, 4 Gt of methane would result in a burden of 10 Gt of methane. When applying a GWP of 105 times carbon dioxide, this would result in a greenhouse effect equivalent to 1050 Pg of carbon dioxide over 20 years.

In conclusion, a release of 1 Gt of methane in the Arctic would be catastrophic and the methane wouldn't go away quickly either, since this would be likely to keep triggering further releases. While some models project rapid decay of the methane, those models often use global decay values and long periods, which is not applicable in case of such abrupt releases in the Arctic.

Instead, the methane is likely to stay active in the Arctic for decades at a very high warming potential, due to depletion of hydroxyl and oxygen, while the resulting summer warming (when the sun doesn't set) is likely to keep triggering further releases in the Arctic.

2 comments:

Sam can you clarify Gt Carbon relative to CH4 volume and mass v CO2e both at Sea and on land permafrost areas because if Global Warming Potential of CH4 is used in Gt figures it doesn't make sense since warming potential depends on time from release and local conditions to reduce it. Amongst other things like induced secondary reactions like in water in lower Stratosphere from its breakdown causing confusion. Thanks.

Yes, the above page is a bit confusing; it was originally posted at knol in 2011 and put here because knol was discontinued. It was updated in January 2012 and should be updated again, but it contains so many useful things that I decided to leave it as is for reference purposes. For updates, have a look at How much time is there left to act and Methane in the Arctic.

Videos

Global temperatures are rising fast. In the Arctic, temperatures are rising even faster (interactive charts below and right). For 2010 and 2011, NASA recorded anomalies of over 2°C at higher latitudes (64N to 90N), with anomalies of over 3°C at latitudes 79N and 81N in 2010.

For November 2010, anomalies of 12.5°C were recorded at latitude 71N, longitude -79 (Baffin Island, Canada). At specific moments in time and at specific locations, anomalies can be even more striking. As an example, on January 6, 2011, temperature in Coral Harbour, located at the northwest corner of Hudson Bay in the province of Nunavut, Canada, was 30°C (54°F) above average.